High-Speed Downlink Packet Access (HSDPA) is a 3G (third generation) mobile telephony communications protocol which allows networks based on Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity and lower latency to end users. HSDPA is an integral feature of Release 5 of the 3rd Generation Partnership Project (3GPP) specification and supports downlink speeds of up to 14.4 Mbit/s and round trip delays of 70 ms. To support HSDPA, the High-Speed Downlink Shared Channel (HS-DSCH) has been added to the UMTS specification. HS-DSCH is the transport channel carrying the user data with HSDPA operation.
HS-DSCH lacks two fundamental features of other wideband Code Division Multiple Access (WCDMA; wideband CDMA) channels—variable spreading factor and fast power control. Instead, it gains an improvement in downlink performance through the utilization of adaptive modulation and coding (AMC), a fast packet scheduling algorithm at the base station, and fast retransmissions from the base station to the mobile station receiver, known as hybrid automatic repeat-request (HARQ).
HARQ processes use incremental redundancy (IR) and chase combining, where data may be transmitted multiple times using different coding techniques. When a corrupt packet is received, a decoder at the receiver combines it with the retransmitted packets in order to correct the errors as efficiently as possible. Even if the retransmissions are corrupted, the combined decoding efforts can still yield an error-free packet.